[SFDXA] [DCARC] SFDXA May Meeting - May 5th: RF Exposure, MPE and SAR

[Kai]

Hi Ken
Wires antennas that are half wave or shorter, no matter if fed in the center or 
end, will have dipole gain (2.1 dB). If longer, there will be multiple lobes, 
but no lobe would exceed dipole gain. Measure the compliance distance from the 
CLOSEST part of the antenna. Always use the ground factor (YES to include 
ground).
Remember that this would give a conservative result. If the distance is 
reasonable, you are done. If too conservative, reduce the average power by 
applying a (for example) 50% TX/RX factor, and a modulation factor (like 25% for 
SSB, 44% for CW, but 100% for FT8).
Kind regards
Kai, KE4PT

On 5/6/2021 10:51, Ken WA2MZE wrote:
> How does one guess what the gain figure for various wire antennas are?  Such 
> as multi-band dipoles (using a tuner), end fed half wave antennas (80 or 40 
> meter half wave on higher bands), and do you measure distance from the feed 
> point, or the antenna center (as you would for a dipole, radiation pattern 
> should be the same as for a dipole, the feed point only affects the impedance 
> to the feed line)?????
>
>
>
> On 5/5/21 9:26 PM, Kai KE4PT wrote:
>> This is what I would have presented... if the audio had been better.
>>
>> The topic is: RF Safety Assessment; May 5, 2021
>>
>> We were always required to be compliant to the RF safety rules:
>> See Form 605 - the micro text was always there!
>> "Item (6) Amateur Applicant/Licensee certifies that they have READ and 
WILL 
>> COMPLY WITH Section 97.13(c) of the Commission’s Rules (available at web site 
>> http://wireless.fcc.gov/rules.html) regarding RADIOFREQUENCY (RF) RADIATION 
>> SAFETY and the amateur
>> service section of OST/OET Bulletin Number 65 (available at web site 
>> http://www.fcc.gov/oet/info/documents/bulletins/)."
>>
>> We were just exempt from certain measurements under some conditions (but we 
>> were never exempt from compliance).
>>
>> There is a sub-group of us in the ARRL RFSC who are now tackling a thorny
>> problem regarding compliance measurements from which we were previously 
>> exempt, both MPE and SAR.
>>
>> The standards have not changed; the "new rules" simply eliminate the ham 
>> exemptions.
>> We are working with the FCC and also cooperation with the RSGB in UK on this 
>> problem.
>>
>> Where compliance is to Max Permissible Exposure (MPE) the determination can 
>> be simple:
>>
>>    http://www.arrl.org/rf-safety-publications
>>    http://www.arrl.org/rf-exposure
>>
>> RF Exposure MPE calculators: THEY All WORK THE SAME WAY!!
>> Make sure that your calculator uses the applicable US FCC Standard. Euros and 
>> most other countries use the ICNIRP standard, it may
>> be slightly different than ours.
>> Use the one with the simplest GUI. They all consider MPE ONLY. For example:
>>
>> http://www.lakewashingtonhamclub.org/resources/rf-exposure-calculator/
>>
>> You enter power, antenna gain, frequency and "yes" to include the 
>> ground/environmental reflection.
>>
>> Calculators find power density in W/m, Pd = S*EIRP/(4 pi r^2) then compare Pd 
>> to the MPE standard to get r=compliant distance.
>> where:
>> EIRP is (Ptx) x (antenna gain); S is an average ground/environmental 
>> reflection power multiplier of 2.56 (an accepted EPA figure).
>>
>> ONE CAUTION, despite what the calculator says to do, ALWAYS measure distance 
>> from the closest part of the antenna.
>>
>> Result would be conservative in most cases. MPE compliance distance will be 
>> overstated in most cases.
>> But can be spot on in some cases, like the NZ5N EME station:
>>  2m, 21dBi, EME array in peak direction with 1500 W pointing at moon rise 
>> (i.e. on the horizon): D = 455 ft with ground reflection!!
>>  In other directions, we must guess ... assume side-lobe gain is 0 dBi in 
>> non-peak directions:
>>  D = 41 feet in all non-peak-of-the-beam directions!!
>>
>> If you don't like the conservative result from the simple calculators, 
make a 
>> detailed model in NEC / EZNEC for a more precise answer, less conservative. 
>> Or use the Tables in:
>> OST/OET Bulletin Number 65 (available at web site 
>> http://www.fcc.gov/oet/info/documents/bulletins/).
>> And you can apply mitigating factors like modulation average power and 
duty 
>> cycle to the power you report.
>>
>> WHAT HAS CHANGED:
>> (1) Hams are no longer exempt from an assessment in certain cases based on 
>> power.
>> (2) For distances less than D=WaveLength/2pi MPE is not used, but rather SAR 
>> (specific absorption rate, in W/kg).
>> That works out to be D = 20 - 40 cm (8 - 16 inches at VHF/UHF). Thus 
we have 
>> a thorny problem with hand-held radios, which now must be
>> assessed using SAR!!
>>
>> The FCC limit for public exposure from radios and cell phones is an SAR level 
>> of 1.6 watts per kilogram (1.6 W/kg). For aware hams it is less restrictive.
>>
>> What is SAR? It was my first job at Motorola in 1979. I was to measure 
SAR on 
>> the head of a cadaver from UHF MX hand-helds.
>> SAR is a measure of how much energy is deposited INTERNALLY into a human 
>> body, as compared with MPE (maximum permissible exposure), which is a measure 
>> EXTERNAL to the body.
>> Those MPE calculators are useful at distances, but do not deal with the pesky 
>> near-in SAR cases. They deal only with a simplified MPE
>> compliance.
>>
>> Do watch the presentation that Barry indicated: 
>> https://www.youtube.com/watch?v=_R6MQD-WZiA
>>
>> Kindest regards,
>> Kai Siwiak, KE4PT
>> ARRL RF Safety Committee
>>
>> QUESTIONS?? Please contact Kai at k.siwiak at ieee.org
>>
>>
>>
>>
>>
>>
>>
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